The reduction-oxidation (redox) flow battery is an electrochemical storage device that stores energy in a chemical form and converts the stored chemical energy to an electrical form via spontaneous reverse redox reactions. The reaction in a flow battery is reversible, so conversely, the dispensed chemical energy can be restored by the application of an electrical current inducing the reversed redox reactions. A single redox flow battery cell generally includes a negative electrode, a membrane barrier, a positive electrode, and electrolytes containing electro-active materials. Multiple cells may be combined in series or parallel to create a higher voltage or current in a flow battery. Electrolytes are typically stored in external tanks and are pumped through both sides of the battery. When a charge current is applied, electrolytes lose electron(s) at the positive electrode and gain electron(s) at the negative electrode. The membrane barrier prevents the positive electrolyte and negative electrolyte from mixing while allowing ionic conductance. When a discharge current is applied, reverse redox reactions occur on the electrodes. The electrical potential difference across the battery is maintained by chemical redox reactions within the electrolytes and can induce a current through a conductor while the reactions are sustained. The amount of energy stored by a redox battery is limited by the amount of electro-active material available in electrolytes for discharge, depending on the total volume of electrolytes and the solubility of the electro-active materials.
Hybrid flow batteries are distinguished by the deposit of one or more of the electro-active materials as a solid layer on an electrode. Hybrid batteries may, for instance, include a chemical that plates as a solid on a substrate throughout the charge reaction and its discharged species may be dissolved by the electrolyte throughout discharge. In hybrid battery systems, the energy stored by the redox battery may be limited by the amount of metal plated during charge and may accordingly be determined by the efficiency of the plating system as well as the available volume and surface area to plate.
In a hybrid flow battery system the negative electrode may be referred to as the plating electrode and the positive electrode may be referred to as the redox electrode. The electrolyte within the plating side of the battery may be referred to as the plating electrolyte and the electrolyte on the redox side of the battery may be referred to as the redox electrolyte.
Anode refers to the electrode where electro-active material loses electrons. During charge, the negative electrode gains electrons and is therefore the cathode of the electrochemical reaction. During discharge, the negative electrode loses electrons and is therefore the anode of the reaction. Therefore, during charge, the plating electrolyte and plating electrode may be respectively referred to as the catholyte and cathode of the electrochemical reaction; the redox electrolyte and the redox electrode may be respectively referred to as the anolyte and anode of the electrochemical reaction. Alternatively, during discharge, the plating electrolyte and plating electrode may be respectively referred to as the anolyte and anode of the electrochemical reaction, the redox electrolyte and the redox electrode may be respectively referred to as the catholyte and cathode of the electrochemical reaction.
One example of a hybrid redox flow battery uses iron as an electrolyte for reactions wherein on the negative electrode Fe2+ receives two electrons and deposits as iron metal during charge and iron metal loses two electrons and re-dissolves as Fe2+ during discharge. On the positive electrode two Fe2+ lose two electrons to form two Fe3+ during charge and during discharge two Fe3+ gains two electrons to form two Fe2+:Fe2++2e−Fe0 (Negative Electrode)2Fe2+2Fe3++2e− (Positive Electrode).
The electrolyte used for this reaction is readily available and can be produced at low costs (such as FeCl2). It also has a high reclamation value because the same electrolyte can be used for the plating electrolyte and the redox electrolyte, consequently eliminating the possibility of cross contamination. Unlike other compounds used in hybrid redox flow batteries, iron does not form dendrites during plating and thus offers stable electrode morphology. Further, iron redox flow batteries do not require the use of toxic raw materials and operate at a relatively neutral pH unlike similar redox flow battery electrolytes. Accordingly, it is the least environmentally hazardous of all current advanced battery systems in production.
However, the above system has disadvantages that limit its practicality in commercial applications. One of these disadvantages is the low cycling performance and poor efficiency of these batteries resulting from a discrepancy in the pH ranges at which the negative and redox electrolytes tend to stabilize. To minimize iron corrosion reactions and to increase iron plating efficiency, a pH between 3 and 4 is desired for the iron plating reaction. However, a pH less than 1 is desired for the ferrous and ferric ion redox reaction to promote redox reaction kinetics and to minimize hydroxide formation.
Concentration gradients across the membrane barrier separating the electrolytes can cause electrolyte crossover. The Fe3+ contamination from the redox side (more acidic) to plating side (less acidic) can cause the formation and precipitation of Fe(OH)3. This precipitate can foul the organic functional group of an ion exchange membrane or can clog the small pores of the micro-porous membrane. In either case, membrane ohmic resistance rises over time and battery performance degrades.
The inventors recognized that the formation of the Fe(OH)3 precipitate could be reduced by the addition of chemical chelating agents in the form of organic compounds. These organic compounds could form complex compounds with Fe3+ which has crossed over from redox side to plating side. These complex compounds are soluble in less acidic environment, and thus stabilize the ferric ions. Further, the colors and potentials of these complex compounds change with solution pH. Therefore, by monitoring the electrolyte pH via an optical sensor and/or electrochemical probe, the addition of chemical additives may be metered so as to achieve and maintain the desired pH in the electrolyte to prevent precipitation and preserve coulombic efficiency.